I would be very interested in seeing an email service by Kagi. I don't pay for email right now besides email connected to one of my own websites. I've thought about giving FastMail or Proton a try, but the pain was never big enough to do that. If Kagi Mail was part of my existing Duo plan in some form, I would give it a try.
I send and receive emails via Apple Mail. A fancy UI would be wasted on me, because I will never use a separate website or app for just one email account. I want everything in one place.
What I yearn for is a really intelligent AI system pre-sorting my email and learning from my behavior. If that would work across all my email accounts, it would be amazing. To set up rules manually just doesn't cut it. Apple's own new categorizations are too unreliable. The AI needs to be better than me at sorting my email (personal mail, press releases, newsletters, notifications, solicitations …). Maybe it would learn from me sorting it, deleting it, replying to it. That's something I'm hoping for. But it has to work 120% of the time. It has to be more than flawless. Otherwise I will just revert to doing it manually, because I can't risk overlooking an important message.
Other than that, I want it to play nice with Apple Mail, follow standards, be fast, reliable.
I'm not sure how this would integrate with Kagi Search. I don't feel like I have a lot of overlap between things I get via email and things I search for.